1. Field of the Invention
This invention deals with techniques for bending and possibly for the thermal treatment of glass sheets, used notably for the production of automobile glazing panes. The invention concerns, more especially, those techniques that consist of heating the glass sheets up to their softening temperature, then bending them by application against an upper bending mold by pneumatic means and/or by means of a lower mold, and finally of causing them to undergo a controlled cooling after bending.
2. Description of the Related Art
According to a conventional technique the sheet passes through a horizontal heating furnace, conveyed on a bed of rollers, to be brought into a bending station where it is raised from the rollers, notably by means of a peripheral suction created by a suction box. The upward movement of the sheet terminates on its contact with the curved lower face of the upper bending mold, the curvature of which the sheet more or less follows under the effect of the suction. Shaping is then completed, either under the conjugate action of inertia and gravity by dropping the sheet onto a curved annular lower mold on which the sheet can then be removed towards the toughening station, or by a mechanical pressing operation of the sheet gripped between the upper mold and an annular lower mold. The sheet is then removed to the toughening station on this same mold or on another annular mold specifically adapted for the toughening. Reference should be made, notably, to Patents FR-B-2 085 464, EP-B-241 355, Ep-A-240 418, Ep-A-241 355 and EP-A-255 432.
This technique of taking over the sheet by peripheral suction is particularly advantageous because it makes possible the use of a large number of variants of the bending process, all starting from a first identical step of preshaping against the curved upper mold.
In this way any intervention of mechanical means for lifting is avoided, which means might result in local excess lifting, a cause of inevitable marks that would mar the optical quality of the curved sheets.
For creating a sufficient pressure difference between the two faces of the sheet, a suction chamber is thus constructed, closed at the top by the upper mold, at the bottom by the sheet, and at the sides by the walls of an open-bottomed box usually termed a "skirt", the depressurization usually being provided by air suction pumps.
Different variants relate to the dimensioning and relative positioning of the upper mold with respect to the existing skirt. Thus, from Patent FRB-2 085 464, there are known skirts profiled in such a way as to channel the lateral escapes or leakages. In this case, the upper molds are entirely housed within the skirt, for the purpose of optimizing the efficiency of the suction, notably at the critical instant at which the sheet lifts off.
It has also been proposed, in Patents EP-A-214 355 and EP-A-240 418 to operate with a "short" skirt, the curved face of the upper mold being flush with the bottom of the said skirt, which enables any harmful contact to be avoided between the walls of the skirt and the sheet in the case where the sheet might be slightly off-centered relative to the upper mold.
But whatever the profile of the skirt, the curvature of the mold and their relative positions, it has hitherto always proved essential to provide the curved face of the upper mold, which is intended to be in contact with the sheets, with an intermediate covering which, when stretched, hugs its contours. This covering, due to its flexibility and its smooth surfaces, softens the contact between glass and mold, notably by "smoothing over" the small surface imperfections of the mold which may remain even after careful machining. Furthermore, if it has a velvety nature, it is suitable for absorbing the dust which would otherwise be trapped between mold and glass, leaving its imprint on the softened glass.
Various types of covering fulfil these functions, notably those described in Patents EP-A-312 439, FR-A-2 606 398 and FR-A-2 644 156, these coverings being essentially based upon knitted or woven metal fabrics.
But even if these fabrics are satisfactory in performance, their gradual wear requires that they be periodically and fairly frequently replaced, in view of the mechanical stresses to which they are subjected, particularly where a pressing step is provided, and in view of the very high temperatures (at least 650.degree. C.) reached by the upper mold and by the whole of the bending station, which cause accelerated oxidizing of the metal of said coverings.
These coverings are currently fixed in a detachable manner to the upper part of the upper mold, where means for attaching them under tension are uniformly distributed.
An operator can only deal with the replacement of the intermediate sheet outside the bending station, both on account of the heat and on account of the lack of access from the side to the upper part of the mold which is housed within the skirt, and this becomes all the more difficult as the mold is enclosed within the skirt. The operator must therefore stop production, disengage the assembly comprising upper mold and skirt from the bending station, both of these being attached, for example, to a removable plate, carry out the change after having allowed the mold and skirt to fall to ambient temperature, and then replace the entirety in the station and await the time necessary for these tools to reach the desired temperature. Apart from the non-negligible time during which production is interrupted by these operations, it is also necessary to allow for an adjustment time necessary following each replacement, due to the fact that it is not entirely possible to guarantee that the operator replaces the assembly comprising plate, skirt and mold exactly where they were before, to the nearest millimeter. A slightly different repositioning leads to a risk of causing slight decentering, notably of the upper mold and skirt relative to a lower annular pressing mold which, even though it may remain within the tolerance limits, would not allow the series of curved sheets to be produced in an identical manner, which is what is always desired.